Auto-collimating digital X-ray system

ABSTRACT

A system and method for automatically collimating X-rays. A digital X-ray system ( 100 ) includes a generator ( 108 ), a sensor unit ( 110 ), a control station ( 112 ), and a preview monitor ( 114 ). The generator ( 108 ) generates X-ray radiation that is captured by the sensor unit ( 110 ) as a digital image and transmitted to the control station ( 112 ). The captured image is displayed on the preview monitor ( 114 ). The generator ( 108 ) includes a collimator ( 212 ) that collimates the generated radiation into a primary beam of X-rays. The size and shape of the primary beam can be adjusted by modifying collimation parameters. A short duration beam of X-rays is generated by the generator ( 108 ) and captured ( 414 ) by the sensor unit ( 110 ). This step is repeated as necessary or desired. The resulting digital images are analyzed ( 418 ) by the control station ( 112 ) to calculate a calibration coefficient. Another short duration beam of X-rays is generated and a reference image is captured ( 514 ). The control station ( 112 ) analyzes the reference image and uses the calibration coefficient to collimate the collimator ( 212 ) to achieve a desired goal, such as radiating all or only part of an object being radiated. Once the collimator ( 212 ) is adjusted, the X-ray image of the subject is exposed.

BACKGROUND

1. Field of the Invention

This invention pertains in general to X-ray systems and in particular toa method and system for collimating the X-rays in such systems.

2. Background of the Invention

Since the discovery of X-rays in 1895 by Wilhelm Roentgen, thepredominant method for capturing an X-ray image has been by exposing aphotographic film. A disadvantage of using photographic film is thatchemical processing must be performed on it to convert the latent X-rayimage into a viewable image. Because of this chemical processing, thereis a delay between when the film is exposed and when the image isviewable. For medical X-ray images taken in the emergency room, forexample, this delay in viewing the image can be critical. Chemicalprocessing of the film, moreover, requires special handling and disposalof the chemicals in order to avoid environmental contamination.

In recent years, computed radiography (CR) has provided a means to takeX-ray images without using photographic film or chemical processing.When using CR, the X-ray image is captured with a photostimulableluminescent plate. The plate is then placed in a special scanner toconvert the captured image into a digital form for subsequent viewing ata computer workstation. Though this technique eliminates the use ofphoto processing chemicals, there is still a significant delayintroduced before the X-ray image is viewable.

More recently, a technique known as digital radiography (DR) has beendeveloped that eliminates both the need for chemical processing and thesignificant delay in producing a viewable image. In DR, a sensor unit,typically an array of amorphous silicon, is used to capture the X-rayimage and produce a digital representation of the image. The sensor unitis coupled to a control station with a cable or other communicationslink. The cable provides power to the sensor unit and transmits digitalcommunication signals between the sensor unit and the control station.Accordingly, the control station receives substantially real-time datadescribing the X-rays detected by the sensor plate.

In all of the above X-ray systems, there is a need to aim the X-raygenerator and collimate the X-rays before capturing the image.Typically, the generated X-rays pass through an adjustable collimatorhaving an aperture that restricts the size and shape of the primary beamof X-ray radiation. Collimation serves to: 1) reduce scatter from X-raysnot required for imaging, thereby improving image quality; and 2) reduceunnecessary X-ray exposure to the patient. Before exposing the X-rayimage, an X-ray technologist manually adjusts the collimator by shininga light located at or near the X-ray generator onto the X-ray sensor orthe patient's anatomical region of interest. The technologist observesthe light reflecting off the sensor plate or patient and manuallyadjusts the aperture in the collimator. Once the light is properlycollimated, the X-ray image is exposed.

In a fast-paced medical environment, such as a hospital emergency roomor a busy clinic, the technologist wastes valuable time and resourceswhen visually collimating the X-ray generator. Moreover, locating thecollimating light near the generator adds an extra level of complexityto the generator design. Therefore, there is a need for an X-ray systemthat simplifies the collimation process. Preferably, the system wouldreduce the time and effort expended by the X-ray technologist tocollimate the X-rays and would not need a visible light to performcollimation.

SUMMARY OF THE INVENTION

The above needs are met by a digital X-ray system (100) having agenerator (108) and a sensor unit (110) in communication with thecontrol station (112). In addition to the sensor unit (110) and controlstation (112), the digital X-ray system (100) preferably includes apreview monitor and operation panel (114) for controlling the X-raysystem (100), an image archiver (116) for storing images captured by thesensor unit (110), a viewing workstation (118) for viewing andmanipulating the images stored in the archiver (116), and a hard copyoutput device (120) for printing the images. In a preferred embodimentof the present invention, the sensor unit (110) captures the digitalX-ray images and transmits the images to the control station (112).Then, the images can be manipulated by the other components of the X-raysystem (100).

The generator (108) preferably includes a radiation source such as anX-ray tube (210) and a collimator (212). The collimator (212) includes aportion blocking the radiation, an adjustable aperture (226) throughwhich a primary beam of radiation passes, and an actuator (230) foradjusting the aperture. By adjusting collimation parameters which affectthe size and shape of the aperture (226), the size and shape of theprimary beam can be adjusted.

The sensor unit (110) is surrounded by a protective cover (310). Withinthe cover (310) are preferably a scintillator (312), a sensor plate(314), and sensor electronics (316). When the sensor unit (110) isexposed to X-rays, the scintillator (312) converts the X-rays intovisible light. The position and intensity of the light is detected bythe sensor plate (314) and stored as a digital image. The digital imageis then transmitted to the control station (112).

In use, the aperture (226) is constricted (410) and a relatively shortduration beam is directed (412) at the sensor unit (110). The resultingdigital image is transmitted (416) to the control station (112). Thisprocess is repeated one or more times with a displaced aperture (226)and the control station (112) calculates (428) a calibration coefficientfrom the captured digital images. This coefficient is preferably stored(430) in the control station (112).

To capture an X-ray image of a subject, a reference image is captured(514) from a short-duration beam and transmitted (516) to the controlstation (112). The control station (112) analyzes (518) the referenceimage and uses the calibration coefficient to adjust the aperture (226)to achieve a desired goal. In one embodiment, the aperture (226) isadjusted to cover the entire field of the sensor plate (314). In anotherembodiment, the aperture (226) is adjusted to restrict the X-ray beamand eliminate unwanted exposure beyond the periphery of the X-raysubject. In yet another embodiment, the technologist views the digitalimage and defines an area of exposure. The control station (112)automatically calculates aperture (226) parameters to restrict the beamto precisely the defined area. Once the aperture (226) is adjusted, theimage of the X-ray subject is captured (526).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a digital X-ray system 100 according to anembodiment of the present invention;

FIG. 2 is a high-level cutaway view of an embodiment of the generator108 of the digital X-ray system 100, including an X-ray tube 210 and acollimator 212;

FIG. 3 is a cutaway perspective illustration of the sensor unit 110 ofthe digital X-ray system 100 according to an embodiment of the presentinvention;

FIG. 4 is a flow diagram illustrating the interactions between thegenerator 108, the sensor unit 110, and the control station 112 whencalculating the calibration coefficient; and

FIG. 5 is a flow diagram illustrating the interactions between thegenerator 108, the sensor unit 110, and the control station 112 whenadjusting the aperture 226 to achieve a desired goal while capturing animage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a digital X-ray system 100 according to anembodiment of the present invention. A sensor unit 110 is incommunication with a control station 112. The control station 112, inturn, is preferably coupled to a preview monitor/operation panel 114, animage archiver 116, a viewing workstation 118, and a hard copy outputdevice 120.

In one embodiment, an X-ray generator 108 generates X-rays that arecaptured as digital data by the sensor unit 110. The digital data aretransmitted from the sensor unit 110 to the control station 112 via acommunications link 122, which in one embodiment is the wireless linkdescribed in U.S. application Ser. No. 09/251,755, entitled WIRELESSX-RAY SYSTEM, filed on Feb. 18, 1999, assigned to the same assignee asthe present invention and hereby incorporated by reference herein. Thecontrol station 112 preferably stores the digital data as a digitalimage in a temporary storage. In addition, the control station 112transmits the digital image to the operation panel 114 which preferablydisplays the image on a preview monitor 114. An X-ray technologistpreferably uses the image displayed on the preview monitor 114 toevaluate the quality of the image, input patient and exposureinformation, if necessary, and direct the image to be stored in theimage archiver 116.

In another embodiment, the control station 112 transmits a list ofpatient names and other information to the sensor unit 110 before theX-ray images are captured. The sensor unit 110 stores the names in amemory and displays the names on an alphanumeric display. The X-raytechnologist matches a displayed name with the patient before taking theX-ray image. When the image is captured, the name of the patient ispreferably encoded with the image according to the Digital Imaging andCommunications in Medicine (DICOM) standard. The X-ray image istransferred to the control station 112.

The image archiver 116 preferably stores hundreds or thousands ofdigital images. An X-ray technologist can use the viewing workstation118 to retrieve an image from the. archiver 116, display the image on adisplay coupled to the viewing workstation 118, and perform assortedimage manipulations, as described, for example, in U.S. patentapplication Ser. No. 09/057,083 now U.S. Pat. No. 6,208,762, entitledOPTIMIZED ENHANCEMENT OF DIGITAL IMAGES, filed on Apr. 8, 1998, assignedto the same assignee as the present application, and hereby incorporatedby reference herein. The hard copy output device 120 prints copies ofimages stored in the image archiver 116.

FIG. 2 is a high-level cutaway view of an embodiment of the generator108, including an X-ray tube 210 and a collimator 212. The illustratedgenerator 108 demonstrates the functionality of a typical generator andis not necessarily representative of an actual X-ray generator. An X-raysystem 100 according to the present invention may use any source ofX-ray radiation and should not be limited to the generator 108 of FIG.2.

The X-ray tube 210 is surrounded by a sealed glass envelope 214. Withinthe envelope are an anode 216 and a cathode 218. The cathode emitselectrons 220, which strike a target 222 on the anode 216. Theinteraction of the electrons 220 with the electrons and positivelycharged nuclei of the target 222 generates X-ray radiation 224. Theenergy of the X-ray emission is controlled by the voltage applied to thetube 210. The X-rays 224 radiate from the target 222 and a portion ofthe X-rays encounter the collimator 212.

The collimator 212 contains a sheet of lead or other material thatblocks the X-ray radiation. An adjustable aperture 226 in the collimator212 allows a “primary beam” 228 of X-ray radiation to pass through thecollimator 212. The size and shape of the primary beam 228 is controlledby adjusting the size and shape of the aperture 226. In one embodiment,an actuator 230 controls the aperture 226. The actuator 230 ispreferably either a servo motor or a stepper motor and is operated by acontrol signal from the control station 112.

FIG. 3 is a cutaway perspective illustration of the sensor unit 110according to an embodiment of the present invention. FIG. 3 illustratesthe top 302 of the sensor unit 110, defined as the side receiving theprimary beam 328 from the X-ray generator 108, and the various lowerlayers of the sensor unit 110. The sensor unit 110 is surrounded by aprotective cover 310. Within the cover 310 are a scintillator 312 and asensor plate 314. As is well known in the art of X-ray detection, thescintillator 312 preferably converts X-ray energy into visible light.The visible light from the scintillator 312 is detected by the sensorplate 314, which digitally records the location and intensity of eachlight flash. In one embodiment of the present invention, the sensor unit110 produces digital X-ray images having 2688×2688 12-bit (4096 grayscale) pixels.

FIG. 4 is a flow diagram illustrating the interactions between thegenerator 108, the sensor unit 110, and the control station 112 whencalculating a calibration coefficient. In the flow diagram, time flowsfrom the top of the diagram to the bottom and horizontal lines representcommunications between the various entities. FIG. 4 illustrates onlymajor interactions between the entities and does not represent everyinteraction.

Initially, the X-ray technologist positions 410 the X-ray generator 108and the sensor unit 110 in the desired positions. In one embodiment ofthe present invention, the sensor unit 110 is stationary and only thegenerator 108 can be positioned. In another embodiment, both thegenerator 108 and the sensor unit 110 are freely positionable.

The X-ray system 100 uses a calibration coefficient to calculate thecollimation parameters. There is a linear relationship between movementof the aperture 226 and movement of the exposure boundary on the sensorunit 110. This relationship is defined by the calibration coefficient asfollows:

 (aperture displacement)×(calibration coefficient)=exposure boundarydisplacement

The calibration coefficient varies with the distance between thegenerator 108 and the sensor unit 110. In a preferred embodiment of thepresent invention, and most X-ray systems, however, this distance isfixed. Therefore, once the calibration coefficient is determined andstored in the control station 112, there is no need for re-calibrationunless the source-sensor distance is changed to a distance for whichthere is no stored coefficient.

To calculate the calibration coefficient, the aperture 226 in thecollimator 212 is moved 412 to a constricted position. Then, thegenerator 108 generates 412 a relatively short duration beam of X-rayradiation towards the sensor unit 110. In one embodiment, the shortduration beam lasts for approximately 20 milliseconds. Preferably, thebeam is generated in response to the technologist entering a command onthe control station 112. The beam is captured 414 by the sensor unit 110and converted into a digital image or other digital format. This imageis transmitted 416 to the control station 112 via the communicationslink 122. The control station 112 analyzes 428 and stores the image.

After the first image is analyzed, the aperture 226 is opened by adisplacement d₁ and the process is repeated. The aperture 226 ispreferably opened automatically by the actuator 230 in response tocontrol signals received 420 from the control station 112. The controlstation 112 analyzes the two images and determines the resultantexposure boundary displacement d₂ corresponding to the aperturedisplacement d₁. The control station 112 calculates 428 and stores thecalibration coefficient as d₂/d₁ and associates it with thesource-sensor distance. These calibration steps can be repeated as manytimes as are necessary or desired to establish one or more calibrationcoefficients.

Once the calibration coefficient is established, the X-ray system 100 iscalibrated for the particular use. FIG. 5 is a flow diagram illustratingthe interactions between the generator 108, the sensor 110, and thecontrol station 112 when adjusting the aperture 226 to achieve a desiredgoal while capturing an image. The X-ray subject can optionally bepresent while the technologist calibrates the X-ray system 100 for theparticular use.

To calibrate the X-ray system 100 for a particular use, the technologistfirst positions 510 the generator 108 and sensor 110 and uses 512 ashort duration X-ray beam to capture 514 a reference image. Thisreference image is transmitted 516 to the control station 112 and thecontrol station analyzes 518 the image and uses the calibrationcoefficient to generate collimation parameters. These parameters aretransmitted 520 to the generator and cause the actuator 230 to adjust522 the aperture 226. These steps can be repeated as many times as arenecessary or desired to achieve a desired collimation.

In one embodiment, the control station 112 constricts the aperture 226and uses the reference image(s) and calibration coefficient to calculatecollimation parameters for widening the beam to cover the entire fieldof the sensor unit 110. In another embodiment, the control station 112uses the reference images to determine where on the sensor unit 110 amaximum (unattenuated) signal was received. The control station 112 usesthe calibration coefficient to determine precisely by how much the beamshould be constricted to eliminate unwanted exposure beyond theperiphery of the X-ray subject or other object being X-rayed. Thisconstriction minimizes the area of the sensor unit 110 receiving X-raysnot passing through the X-ray subject and thereby reduces unwanted X-rayscatter. In a third embodiment, the technologist views the referenceimage captured from the short duration exposure and defines the desiredarea of exposure. For example, the technologist can use a keyboard,mouse, or other input device to define the borders of the desired areaof exposure. The control station 112 uses the calibration coefficient todetermine by how much the beam should be constricted or expanded tocover only the defined region.

Once the collimator 212 is properly adjusted, the technologistpreferably positions the subject to be X-rayed between the generator 108and the sensor unit 110 (if the subject is not already so positioned)and causes the generator 108 to generate 524 a full duration beam. Inone embodiment, the fill duration is approximately 200 milliseconds. TheX-ray image is captured 526 by the sensor unit 110 and transmitted 528to the control station 112 for subsequent processing.

The above description is included to illustrate the operation of thepreferred embodiments and is not meant to limit the scope of theinvention. The scope of the invention is to be limited only by thefollowing claims. From the above discussion, many variations will beapparent to one skilled in the relevant art that would yet beencompassed by the spirit and scope of the invention.

We claim:
 1. A method for collimating radiation, comprising the stepsof: analyzing a plurality of collimation images captured by a sensorunit for capturing an x-ray image based on x-ray radiation from agenerator, wherein in capturing the plurality of collimation images thex-ray radiation is collimated to a plurality of different positions by acollimator for collimating x-ray radiation and wherein in analysis ofthe plurality of collimation images, a calibration coefficient iscalculated for calibration of the collimator, the calibrationcoefficient at least being responsive to a relationship, in theplurality of collimation images, between movement of an aperture in thecollimator and movement of an exposure boundary on the sensor unit; andgenerating a collimation parameter for collimation of the collimator atleast in accordance with the calibration coefficient, a reference imagefrom the sensor unit, and an area defined on the reference image.
 2. Amethod according to claim 1, wherein the analyzing step is for analyzingtwo collimation images from the sensor unit responsive to two exposuresof x-ray radiation collimated to two different positions by thecollimator, and wherein the calibration coefficient is calculatedresponsive to a linear relationship between movement of the aperture inthe collimator and movement of an exposure boundary on the sensor unit.3. A method according to claim 1, wherein the collimation parameter isgenerated such that radiation from the generator covers an entire fieldof the sensor unit.
 4. A method according to claim 1, further comprisingthe steps of: displaying the reference image; and defining an area onthe displayed reference image; wherein the generating step is forgenerating a collimation parameter for radiating the defined area.
 5. Amethod according to claim 1, wherein the generating step comprises thesteps of: analyzing the reference image to determined if radiationextends beyond an object to be radiated; and calculating the collimationparameter for reducing radiation extending beyond the object to beradiated, responsive to a positive determination that radiation extendsbeyond the object to be radiated.
 6. A method according to claim 1,wherein the area defined on the reference image is defined with an inputdevice by a user.
 7. An x-ray system comprising: a sensor unit forcapturing an x-ray image based on x-ray radiation from a generator; anda control unit coupled to the sensor unit and a collimator forcollimating x-ray radiation from the generator; wherein the control unitcomprises: a calibration unit for analyzing a plurality of collimationimages captured by a sensor unit for capturing an x-ray image based onx-ray radiation from a generator, wherein in capturing the plurality ofcollimation images the generator is collimated to a plurality ofdifferent positions by a collimator for collimating x-ray radiation andwherein in analysis of the plurality of collimation images, acalibration coefficient is calculated for calibration of the collimator,the calibration coefficient at least being responsive to a relationship,in the plurality of collimation images, between movement of an aperturein the collimator and movement of an exposure boundary on the sensorunit; and a parameter generation unit for generating a collimationparameter for collimation of the collimator at least in accordance withthe calibration coefficient, a reference image from the sensor unit, andan area defined on the reference image.
 8. An x-ray system according toclaim 7, wherein the calibration unit analyzes two collimation imagesfrom the sensor unit responsive to two exposures of x-ray radiationcollimated to two different positions by the collimator, and calculatesthe calibration coefficient responsive to a linear relationship betweenmovement of the aperture in the collimator and movement of an exposureboundary on the sensor unit.
 9. An x-ray system according to claim 7,wherein the area defined on the reference image is defined with an inputdevice by a user.
 10. An x-ray system according to claim 7, wherein thearea defined on the reference image corresponds to an entire field ofthe sensor unit.
 11. An x-ray system according to claim 7, wherein thearea defined on the reference image is defined to reduce radiationextending beyond an object to be radiated.
 12. A computer-readablemedium storing a computer-executable program, the computer programcomprising instructions for: analyzing a plurality of collimation imagescaptured by a sensor unit for capturing an x-ray image based on x-rayradiation from a generator, wherein in capturing the plurality ofcollimation images the generator is collimated to a plurality ofdifferent positions by a collimator for collimating x-ray radiation andwherein in analysis of the plurality of collimation images, acalibration coefficient is calculated for calibration of the collimator,the calibration coefficient at least being responsive to a relationship,in the plurality of collimation images, between movement of an aperturein the collimator and movement of an exposure boundary on the sensorunit; and generating a collimation parameter for collimation of thecollimator at least in accordance with the calibration coefficient, areference image from the sensor unit, and an area defined on thereference image.
 13. A computer-readable medium according to claim 12,wherein the analyzing instruction is for analyzing two collimationimages from the sensor unit responsive to two exposures of x-rayradiation collimated to two different positions by the collimator, andthe calibration coefficient is calculated responsive to a linearrelationship between movement of the aperture in the collimator andmovement of an exposure boundary on the sensor unit.
 14. Acomputer-readable medium according to claim 12, wherein the collimationparameter is generated for radiation to cover an entire field of thesensor unit.
 15. A computer-readable medium according to claim 12,wherein the generating instruction comprises instructions for: analyzingthe reference image to determine if radiation extends beyond an objectto be radiated; and calculating the collimation parameter for reducingradiation extending beyond the object to be radiated, responsive todetermination that radiation extends beyond the object to be radiated.16. A computer-readable medium according to claim 12, wherein the areadefined on the reference image is defined with an input device by auser.